#include <bits/stdc++.h>
using namespace std;
map<char, int> mp;
int main() {
  string s;
  cin >> s;
  for (char fu : s) {
    mp[fu]++;
  }
  for (char fu = 'a'; fu <= 'z'; fu++) {
    if (mp.count(fu) == 0) {
      cout << fu << endl;
      return 0;
    }
  }
  return 0;
}